This invention relates to microcellular radio transmission systems, and is particularly directed to dynamic radio frequency scanning, planning, and channel allocation for microcellular systems coexisting in a macrocellular environment.
The last decade has witnessed an explosion in the deployment of analog and digital cellular radio communication systems across the globe. Such systems provide spectrum efficient, low-power wireless communication between mobile or fixed communications terminals (xe2x80x9cmobilesxe2x80x9d) and fixed radio transceivers (xe2x80x9cbase stationsxe2x80x9d) dispersed throughout the service area of a macro cellular system (xe2x80x9cmacrosystemxe2x80x9d). Typically, each macrosystem base station maintains a permanent communications link with a mobile telephone exchange (MTX), which is responsible for wireless call processing as well as interfacing the mobiles to the Public Switched Telephone Network (xe2x80x9cPSTNxe2x80x9d).
In narrowband cellular macrosystems, such as defined in the IS-136, IS-54B, and GSM standards, RF spectrum efficiency is realized through geographic distribution and reuse of a finite number of signal-bearing carrier frequencies within the macrosystem service area. Conventionally, this is accomplished through geographically segmenting the service area into a number of macrocells, each containing at least one base station. Within each macrocell, wireless communications are established between mobiles within the macrocell and the macrocell base station(s) using only a portion of the radio frequency spectrum or operational band allocated to the macrosystem. In so doing, each macrocell may be statically or dynamically allocated a subset from the pool of more than 400 defined frequency pairs within the macrosystem""s operational band. Adjacent macrocells are assigned non-overlapping frequency channels to maximize traffic loading without producing co-channel interference. However, at or beyond the reuse distance of a given macrocell (which is dictated by such factors as base station/mobile transmitter power and receiver sensitivity specifications, macrocell size, and terrain), a set of partially or fully overlapping frequency pairs may be allocated.
Narrowband cellular radio is inherently scaleable, and has proven to be a reliable, wireless communications system offering relatively low fixed-end and infrastructure costs when smaller geographical areas are to be serviced. Network communications companies have also recognized the market need for private wireless communications having network functions tailored to the specific requirements of individual businesses, government, and private institutions. Hence, it has become popular to deploy a private microcellular radio communication system or xe2x80x9cmicrosystemxe2x80x9d within the macrosystem service area. Such a microsystem will typically be integrated into a private branch exchange (xe2x80x9cPBXxe2x80x9d) as part of a private communications network and can be conveniently deployed within an office building, campus or worksite to enable wireless communications among the occupants thereof. Potentially each subscribing mobile positioned within the microsystem""s service area can register with this private network through the microsystem""s base station(s) and emulate a desktop terminal served by the PBX.
Representative private microcellular networks have been developed by Northern Telecom Limited, the assignee of the present invention. See, for example, U.S. Pat. Nos. 4,771,448 to Koohgoli, et al. and 5,537,610 to Mauger, et at One of the hallmarks of these and other known microsystem approaches is the ability for both the microsystem and macrosystem to utilize the same mobile unit. Another is the nearly seamless handoff operation a subscribing mobile undergoes when moving from the macrosystem to the microsystem (such as when a mobile""s user walks into the office building) and vice versa. Yet another feature is the ability for the mobile to remain registered on the macrosystem and potentially make and receive macrosystem calls while in the microsystem""s coverage area but not registered on this microsystem.
To accommodate these features while conserving scarce frequency spectrum resources, a microsystem must be able to re-use some of the traffic and control frequencies allocated to the overlying macrosystem. But, in order for the microsystem to xe2x80x9cpeacefullyxe2x80x9d coexist with the macrosystem, the microsystem must be able to quickly obtain and relinquish frequency resources without inducing co-channel interference or otherwise disrupting macrosystem operations. Traditionally, macrosystem spectrum resources were statically assigned to the macrocells, and so microsystem planners could routinely predict and reserve frequencies allocated to the macrosystem but left unassigned to the macrocell(s) in which the microsystem was located. However, as macrosystem traffic demands increasingly strain capacity, macrosystem planners have responded by all-too-frequently adapting macrocell frequency allocations to varying use patterns, thereby making cooperative microsystem resource planning difficult, if not impossible to implement.
In response, an automatic frequency allocation system has been proposed in laid-open PCT application WO 96/31075. The disclosed system will enable the microsystem to scan the uplink subrange of frequencies allocated to the overlying macrosystem through periodically placing an idle microcell base station transceiver in a locate receiver mode. Once the frequency subrange is scanned, measurement data associated with each scanned frequency is evaluated by the microsystem and a set of available frequencies is determined for use in microsystem communications.
However, in this system, no provision has been made to efficiently apprise registered mobiles within range of the microsystem of backup control channel information as well as control information for the overlying cells of the macrosystem and any competing Microsystems, particularly when such mobiles are being actively serviced by the microsystem. This is so even though the microsystem is attempting to cooperatively share a common operational band with the overlying macrosystem and any competing microsystems.
Therefore, it would be desirable for a microsystem to manage such control information at a local level which can be made conveniently accessible to all registered mobiles being serviced by the microsystem. Furthermore, it would be advantageous if, in so doing, the microsystem would be able to exploit yet remain compatible with existing planning information issued by the overlying macrosystem, including, but not limited to macrosystem neighbor list communications. Finally, it would be advantageous if microsystem-centric control channel information could be presented and assimilated by the registered mobiles without substantial modification to the mobiles themselves.
In light of the above shortcomings and desires, the present invention is directed to a method for assembling a microsystem neighbor list to define control channel usage within the operational band being used by the microsystem, the overlying macrosystem, and any competing Microsystems. The present invention is also directed to a frequency planning controller and microsystem utilizing this method, as well as a computer program product defining the same.
Preferably, assembly of the microsystem neighbor list includes defining preferred and non-preferred members. In the below-described embodiments, the preferred member(s) of the microsystem neighbor list will include at least one control channel defined within the common operational band which is determined to be available for use by the microsystem. In the typical case, more than one preferred member will be defined in order to provide backup control channels should interference be detected.
According to one embodiment of the invention, potential preferred members (and suitable control channel candidates) of the microsystem are identified through scanning a set of control channel candidates to obtain measured RSSI signal characteristics for each, comparing the measured signal strengths against a tolerance threshold, and filtering out those control channel candidates whose measured RSSI characteristics indicate a likelihood of use by the overlying macrosystem or a competing microsystem. Thus, those control channel candidates who exhibit sufficiently clean signal characteristics will be chosen to be preferred members of the microsystem neighbor list, and consequently, as active/backup control channels for the microsystem.
The aforementioned set of control channel candidates may include pre-designated candidates, or can also include any frequency pair defined within the operational band.
Further, preferably, the non-preferred members of the microsystem neighbor list will include the control channels being used by adjacent macrocells of the overlying macrosystem, or even competing Microsystems. This information may be conveniently obtained by acquiring the system-wide macrosystem or cell-specific macrocell neighbor list issued by the overlying macrosystem. In such case, the preferred members of the macrosystem or macrocell neighbor list will become the non-preferred members of the microsystem neighbor list.
According to the present invention, the so-assembled microsystem neighbor list comprising preferred and non-preferred members may be broadcast throughout the service area of the microsystem in order to apprise all registered mobiles of active/backup control channel information for the microsystem, thereby improving control channel allocation flexibility and control channel recovery operations. The broadcast microsystem neighbor list also provides control channel information for competing Microsystems (if any) or adjacent macrocells, thereby improving switch-over to such systems when desired by the mobile operator.
Though not required, it is advantageous to convert the assembled microsystem neighbor list into a PSP-POF compatible table for transmission to the registered mobiles. Doing so ensures proper recognition of the control channel information for both the microsystem as PSP entries and the overlying macrosystem/competing microsystem(s) as POF entries, at least from the microsystem""s perspective, without substantive modification to the PSP-POF handling routines or logic contained on the registered mobiles. Of course, in such a configuration, use of PSP-POF compatible mobiles (such as those specified by the IS-136 TDMA cellular communications standard) is believed required.
An additional aspect of the invention is maintainability of the microsystem neighbor list after assembly. The preferred member(s), particularly the active control channel, can be routinely checked to see if co-channel interference has been perceived thereon, and if so, such preferred member(s) can be removed from the neighbor list and replacement member(s) can be acquired. Furthermore, preferably the non-preferred members can be periodically verified to keep microsystem neighbor list data reasonably fresh and accurate of control channel use within the common operational band.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.